Optoelectronic sensors for detecting and locating objects are valuable in a wide variety of fields; one example is industrial automation. In order to understand the operation, capabilities, and limitations of such sensors, consider a specific example application—printing alphanumeric characters on labels in a high-speed production bottling line. In a typical such application, a sensor detects the presence of a bottle at a predetermined point as it moves down the production line, and produces a signal that tells a printer to start printing when a bottle is located at that point.
If a signal is produced when a bottle is at the proper location, desired alphanumeric characters are printed on the bottle's label in a desired location. If a bottle is not detected, however, its label will not be printed, a potentially costly mistake. If a signal is produced when no bottle is present, the printer may spray ink into the air, which may have adverse effects on the production process. If a signal is produced when a bottle is present but not in the proper location, the characters will be printed in the wrong location on the label.
From this example it can see seen that an optoelectronic sensor for detecting and locating objects should detect only the desired objects, and only in the desired location.
Photoelectric sensors, which comprise a type of optoelectronic sensor, have long been used to detect and locate objects. Such sensors typically operate by emitting a beam of light and detecting light received. They come in a variety of configurations suitable for a variety of applications, including the following:
In one configuration an emitter and a receiver are placed at opposite ends of a path, so that anything crossing the path that is not transparent breaks the beam of light; an object is detected when the receiver sees very little light. The placement of the emitter and receiver determines the path and thereby the location at which an object is detected. The application is constrained to insure that only desired objects cross the path, and so that determining the location of an edge of the object is all that is needed. In the bottling application, for example, this would mean that the label is in a fixed position relative to the edge of the bottle.
In a second configuration an emitter and receiver are placed in one location, with a retro-reflector placed at the opposite end of a path that reflects the beam from the emitter back to the receiver. This configuration is similar to the previous, but is more convenient to install because all of the required wiring is done at only one end of the path instead of at both ends. However, if the objects to be detected are reflective, misdetection and/or mislocation errors may occur.
In a third configuration an emitter and receiver are placed in one location, and the emitter emits a focused beam of light so that anything sufficiently reflective crossing in front of the beam reflects it back to the receiver. An object is detected when the receiver sees an amount of light above some predefined threshold. The placement of the emitter/receiver assembly determines the location of the beam and thereby the location at which an object is detected. The use of a focused beam makes this location relatively precise, and reduces the chances of misdetecting objects in the background because the beam will be out of focus. The objects and their environment are constrained so that the reflected light exceeds the threshold only when desired objects are in the desired location.
A fourth configuration is a variation of the third wherein a diffuse beam of light is used instead of a focused beam. This makes it easier to detect objects whose positions are not well constrained, but decreases the precision of the location at which objects are detected and increases the chances that a detection will occur when no desired object is in front of the beam.
Photoelectric sensors of these types include those manufactured and sold by Banner Engineering Corp., of Minneapolis, Minn., for example their MINI-BEAM® line.
Photoelectric sensors typically provide a simple signal to indicate that an object has been detected, and to indicate its location. Such a signal has two states, which might be called “present” and “absent”. In the first and second configuration, for example, the signal would be in the “present” state when little light is detected by the receiver; in third or fourth configuration, the signal would be in the “present” state when light above a threshold is detected by the receiver.
An object is detected when the signal is in the “present” state. A moving object is located by the time of a transition from “absent” to “present” on the signal—the object is located in a known, predetermined position, called herein a reference point, at the time of the transition. For any of the four photoelectric sensor configurations described above, the reference point is determined by the position of the beam and is adjusted by physically moving the sensor. Note that as used herein the reference point refers to the desired location of the object; the actual location of the object at the signal time may differ for various reasons as described herein.
Usually a photoelectric sensor is used to detect specific objects and locate them at a specific position, for example to detect bottles moving down a conveyer belt and indicate the time at which the leading edge of such a bottle has reached a certain reference point, for example in front of a printer. No sensor is a perfect judge of object presence and location, and failures will result if a desired object is not detected, if some other condition, such as an unexpected object or light source, results in a false detection, or if a desired object is detected but mislocated.
In order to increase the reliability of detection and location, a photoelectric sensor may employ one or more techniques. The different configurations described above, for example, allow different tradeoffs between missed objects, false detection, and other factors that can be matched to the needs of a specific application. For example, a retro-reflective configuration is unlikely to miss an object but requires mounting two elements, the sensor and the reflector, in suitable locations. A focused beam is less likely to detect an unexpected object because it has a narrow range of focus.
A sensitivity adjustment is typically used as another tradeoff between missed objects and false detection for each sensor configuration. In the focused beam configuration, for example, the sensitivity adjustment determines the predefined detection threshold. Increasing the threshold makes false detections less likely but missed objects more likely; decreasing the threshold makes the opposite tradeoff.
In addition, sensors may employ a suitably modulated beam of light so that stray light is unlikely to cause a misdetection. For example, the beam can be pulsed on and off at a high frequency, and the receiver is designed to only detect light pulsed at that frequency.
A commonly used term of art for photoelectric sensors is background suppression, which describes various means for ignoring reflections of the emitted beam by objects in the background (i.e. beyond some predetermined distance), so that misdetection can be made less likely. U.S. Pat. No. 5,760,390, for example, describes such a device as well as providing information on prior art devices.
Photoelectric sensors are well-suited to object detection and location, and are widely used, but they have a variety of limitations.
Such sensors have a very limited ability to distinguish desired objects from other conditions such as unexpected objects, variations in object reflectivity, confusing markings or features on the objects, and the like. Generally all the sensor can measure is how much light it's receiving. A gray object at 5 centimeters distance, for example, might reflect as much light back to the receiver as a white object at 10 centimeters, even if the beam is somewhat out of focus at 10 centimeters. Considerable care must be taken to insure that the presence of a desired object at a desired location, and no other condition, results in an amount of light received that is above or below a predetermined threshold. This limits the ways in which such sensors can be installed, the nature of objects to be detected and located, and the manner in which such objects are presented to the sensor, among other concerns. Furthermore, generally only edges of objects can be detected and located; it is difficult and in many cases impractical to detect and locate markings or other features on the object.
With a photoelectric sensor, establishing or adjusting the reference point generally involves physically moving the sensor. Such an adjustment is challenging to make and may have undesirable consequences if not made carefully. Furthermore, it is difficult to establish a reference point with high precision.
Photoelectric sensors have an inherent delay between the time that an object crosses the reference point and the time that the signal transition occurs. This delay is commonly called latency or response time, and is typically of the order of hundreds of microseconds. An object will move away from the reference point during this delay, by a distance that increases as object velocity increases. Furthermore, since a reference point is usually established at installation time using stationary objects, the actual location of the object at the signal time will always be different in production use where the objects are moving. If the latency and velocity are known it may be possible to compensate for the latency, but doing so adds complexity and cost, and might not always be practical.
The latency of a photoelectric sensor can generally be reduced by giving up some reliability of detection. The less time the sensor has to make a detection decision, the less reliable the decision will be.
Photoelectric sensors have an inherent uncertainty in the signal transition time, typically referred to in the art as repeatability, and also typically of the order of hundreds of microseconds. This translates to uncertainty in the location of the object at the signal time, where the uncertainty is proportional to object velocity.
The combination of physical positioning, latency, and repeatability limit the accuracy of a photoelectric sensor in locating objects.
Recently a new class of optoelectronic sensors have been developed, described in pending U.S. patent application Ser. No. 10/865,155 filed Jun. 9, 2004. These sensors, called therein vision detectors, address some of the limitations of photoelectric sensors. A vision detector uses a two-dimensional imager (similar to those used in digital cameras) connected to a computer or like device that analyzes the digital images to make detection and location decisions.
Vision detectors provide an ability to distinguish desired objects from other conditions that far surpasses that which can be achieved with photoelectric sensors. This ability is achieved by using two-dimensional brightness patterns to detect and locate objects. Establishing a reference point can be done electronically by means of a human-machine interface. These devices, however, exhibit significant latency, on the order of several milliseconds, and repeatability that, even under favorable conditions, is no better than that of photoelectric sensors.
Vision detectors, furthermore, are very expensive compared to photoelectric sensors, and far more complex to set up for a given application.